The environment contains a variety of infectious microbial agents, such as viruses, bacteria, fungi and parasites, any one of which can cause pathological damage to the host organism. Consequently, most organisms, such as mammals, i.e. humans, have developed an immune system. The immune system is divided into two functional divisions, the innate immune system and the adaptive immune system.
The innate and adaptive immune system consists of a variety of molecules and cells distributed throughout the body. The most important cells are leukocytes. Leukocytes are categorized as phagocytes, including polymorphonuclear neutrophils (PMNs), monocytes and macrophages, and lymphocytes, which mediate adaptive immunity.
Inflammation is the body's response to invasion or an injury, such as an invasion by an infectious microbial agent and includes three broad actions. First, the blood supply is increased to the area. Second, capillary permeability is increased, thereby permitting larger molecules to reach the site of infection. Third, leukocytes, particularly PMNs, migrate out of the capillaries and into the surrounding tissue. Once in the tissue, the PMNs migrate to the site of infection or injury by chemotaxis. These events manifest themselves as inflammation. Examples of conditions which cause these reactions to occur include clamping or tourniquet vessel-induced ischemia reperfusion injury, chronic inflammatory conditions such as asthma, rheumatoid arthritis, and inflammatory bowel disease, as well as autoimmune diseases.
Aberrant activation of phagocytic cells, in particular neutrophils, leads to the generation of superoxide anion, which when released to the extracellular milieu can evoke damage to surrounding tissues. Reactive oxygen species derived from neutrophil oxygen burst can play a deleterious role in generating secondary products that lead to loss of function. During surgery, in particular clamping of vessels, there is clear evidence that reperfusion following the release of the clamp involves neutrophil-derived mediators. The neutrophil-derived oxygen radicals and other toxic products that are normally intended for killing of microbial agents once they spill over into the surrounding tissue can lead to second organ injury, most notably in the lung and cardiac tissues, sequelae which are observed following ischemia reperfusion injury (Welbourn et al., Brit. J. Surg. 1991; 78:651-655).
Once at the site of infection, PMNs perform phagocytic and degradative functions to combat the infectious agent. As part of the response to the infectious agent, PMNs generate superoxide anions, reactive oxygen species (ROS) to kill infested material and adhere to epithelial cells of mucosal surfaces or vascular endothelial cells of the blood vessels. As a consequence, the host can experience undesirable side effects during the elimination of the infectious agent such as, pain, swelling about the site, and nausea.